This invention relates to the production and processing of higher hydrocarbons, specifically waxes, useful as coating materials, in candles and in a wide variety of applications including food and drug applications which require high purity wax. More particularly, this invention relates to the production of high paraffin wax products produced by the reaction of carbon monoxide and hydrogen, the Fischer-Tropsch process. Still more particularly this invention relates to a catalytic process whereby raw Fischer Tropsch wax is subjected to a mild hydrotreating process yielding a high purity, hydrocarbon wax product of desired hardness without the need for further processing.
The catalytic production of higher hydrocarbon materials from synthesis gas, i.e. carbon monoxide and hydrogen, commonly known as the Fischer-Tropsch process, has been in commercial use for many years. Such processes rely on specialized catalysts.
The original catalysts for Fischer-Tropsch synthesis were typically Group VIII metals, particularly cobalt and iron, which have been adopted in the process throughout the years to produce higher hydrocarbons. As the technology developed, these catalysts became more refined and were augmented by other metals that function to promote their activity as catalysts. Such promoter metals include the Group VIII metals, such as platinum, palladium, ruthenium, and iridium, other transition metals such as rhenium and hafnium as well as alkali metals. The choice of a particular metal or alloy for fabricating a catalyst to be utilized in Fischer-Tropsch synthesis will depend in large measure on the desired product or products.
The products from hydrocarbon synthesis must be useful in a variety of applications. The waxy product a hydrocarbon synthesis, particularly the product from a cobalt based catalyst process contains a high proportion of normal paraffins. It is generally known to catalytically convert the paraffin wax obtained from the Fischer-Tropsch process to lower boiling paraffinic hydrocarbons falling within the gasoline and middle distillate boiling ranges, primarily by hydrogen treatments e.g. hydrotreating, hydroisomerization and hydrocracking. However, new markets continue to expand in demand for petroleum and synthetic waxes. The varied and growing uses for the waxes, e.g. food containers, waxed paper, coating materials, electrical insulators, candles, crayons, markers, cosmetics, etc. have lifted this material from the by-product class to the product class in many applications.
Stringent requirements are set by regulatory authorities such as the FDA in the United States and the SCF in the European Union, which a wax should meet, particularly if the wax is to be used in food and drug applications. Further, it is a demanding task for the crude oil refiner to meet those requirements. Petroleum waxes derived from crude oil often have dark color, poor odor and numerous impurities requiring significant further refining, particularly when wax is to be used in food and drug applications which require highly refined wax in order to satisfy the regulatory authorities. The presence of sulfur, nitrogen and aromatic species, which induce a yellowish or brownish color, are undesirable in that they may present considerable health risks. Intensive wax refining techniques are required to improve thermal and light properties, ultra-violet stability, color, storage stability and oxidation resistance of the end products. Typically, such waxes are subjected to wax decolorization processes commonly denoted as wax finishing. Such methods are part of a time consuming and costly process and have a detrimental effect on opacity which is desirable in a number of applications where superior thermal and light properties, ultraviolet stability, color and storage stability are desired. These applications include, but are not limited to coating materials, crayons, markers, cosmetics, candles, electrical insulators and the like as well as food and drug applications.
Waxes prepared by the hydrogenation of carbon monoxide via the Fischer-Tropsch process have many desirable properties which make them superior to petroleum waxes in numerous respects. They have high paraffin contents and are essentially free of any sulfur, nitrogen and aromatic impurities found in petroleum waxes. However, untreated Fischer-Tropsch waxes may contain a small but significant quantity of olefins and oxygenates (e.g. long chain primary alcohols, acids and esters) which can cause corrosion in certain environments. Therefore, Fischer-Tropsch waxes typically undergo some type of hydroprocessing to obtain high purity.
In addition, Fischer-Tropsch waxes are harder than conventional petroleum waxes. The hardness of waxes and wax blends as measured by needle penetration can vary considerably. Hardness for waxes is generally measured by the needle penetration test ASTM D 1321. In general, the hardness of Fischer Tropsch waxes is an advantage since there exists a shortage of high-grade hard paraffin waxes. However, such hardness could limit the usefulness of untreated Fischer-Tropsch waxes in certain applications. Thus, it would be desirable to provide a process by which the hardness of these waxes could be efficiently adjusted to within desired ranges during hydroprocessing.
The present invention is directed to a mild hydrotreating process which removes the oxygenates and olefins and any aromatic species which may be present from a raw Fischer Tropsch wax while simultaneously reducing the hardness, thereby limiting or eliminating the need for further processing.
The process involves producing a raw Fischer-Tropsch wax in a hydrocarbon synthesis process and then passing the raw wax over a hydroisomerization catalyst under mild conditions such that chemical conversions (e.g., hydrogenation and mild isomerization) take place while less than 10% boiling point conversion (hydrocracking) occurs, thus preserving overall yield of wax product.
In one embodiment of the present invention, a raw Fischer-Tropsch wax is formulated via hydrocarbon synthesis and the wax hardness, as defined by ASTM Standard Test Method for Needle Penetration of waxes (ASTM D-1321), is adjusted to within a region preferred for end use applications, while simultaneously removing undesirable impurities, such as oxygenates (e.g., primary alcohols), olefins, and trace levels of aromatics if they are present.